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GUEST BLOG: To Reduce Weeds and Improve Lawn? Apply Calcium

The lawn on the left received applications of high-calcium limestone and gypsum; the lawn on the right did not.

The lawn on the left received applications of high-calcium limestone and gypsum; the lawn on the right did not.

For years I’ve seen what the additions of high-calcium limestone can do for a lawn to decrease weed pressure and create overall soil health. To help explain the process in depth, I asked agronomist Craig Dick of to prepare this guest blog:


By Craig Dick
Almost everything you do to your yard can destroy the quality of your soil. Applying synthetic nitrogen decreases organic matter and soil carbon, leaches calcium, causes acidity, and can reduce beneficial soil bacteria. Mowing equipment, walking and playing on the lawn causes compaction. Planting only one variety of grass may increase diseases and insects. Spraying herbicides/insecticides can reduce beneficial bacteria and insects and increase soil compaction problems.

So, how do we improve soils in our landscape high quality that will help us maintain lush landscapes?

It’s funny that calcium is rarely considered as a plant nutrient at all by academia. It is true that NPK (Nitrogen, Phosphorus and Potassium) is used in greater percentages in plants than calcium, but calcium is used more by weight and volume than any other nutrient. Calcium, which is commonly found in limestone and gypsum, should be considered the most important nutrient, and more than simply a tool to move the pH scale. Calcium plays a major role in the physiology of the plant, strengthening its physical structure, increasing nutrient uptake and protecting it from disease.

Plant available calcium determines the uptake of all other nutrients into the plant. It is the carrier of all other nutrients into the plant. As calcium content in the plant drops, so can the protein, mineral and energy levels of the plant, resulting in disease and insect infestations. According to Plant Pathology, by George Nicholas Agrios, calcium reduces the severity of rhizoctonia (brown patch), sclerotium (white mold), fusarium (snow mold) and nematodes. The Plant Pathologist’s Pocketbook states, “Calcium generally enhances resistance against disease.” Adequate plant nutrition makes plants more tolerant and resistant to disease.

Calcium is supremely important in the soil. Benefits of calcium in the soil include the reduction of soil compaction, increased water infiltration, reduced soil erosion and phosphorus loss, which provides a better environment for the proliferation of beneficial bacteria. Some research even suggests that calcium plays a role in reducing weed populations.

A common misconception is that if the pH is high, adequate calcium is present. Soil pH is only a measure of potential hydrogen (hence pH) and has nothing to do with calcium. High levels of magnesium, iron, sodium, and potassium can increase soil pH.

In the 1940s Dr. William A. Albrecht, working in Missouri, concluded through his own investigations that plants require a soil with a high Ca saturation for optimal growth. At about the same time, the idea of cation ratios was originally proposed by Firman Bear and coworkers in New Jersey as a method of reducing or enhancing nutrient update. The study of the ratios between calcium and magnesium is now used by many private soil-testing laboratories and consultants for the interpretation of soil analytical data.

Our experience has shown that as long as the calcium and magnesium parts per million are sufficient in the soil (2000+ for Ca, 250+ for Mg) a Calcium to Magnesium ratio from 7:1 and up to 20:1 should offer a soil with better structure, better aeration, and better productivity — in short a much healthier soil resulting in a much healthier lawn with little need for chemical intervention.

Calcium is the element that causes the soil particles to move apart for aeration and drainage. Magnesium makes the particles stick together. One soil consultant has determined that in some soils the excess magnesium is held as trimagnesium ortho phosphate, Mg3(PO4)2-22H2O. Notice that the last part of the formula is twenty-two molecules of water.

The magnesium ions sitting on the soil surface hold 50 percent more water than calcium. This water is held so tightly by the soil it doesn’t drain and plants cannot access it. This bound water tends to weaken the forces that hold soil particles together resulting in less aggregate stability and greater dispersion of soil particles. This reduces water infiltration rates and hydraulic conductivity — otherwise known as drainage.

High magnesium soils tend to swell when wet and become very hard when dry, often forming a hard surface crust. High magnesium soils will typically have more sodium cations attached to the clay as well. Having high magnesium and sodium causes the clay particles to disperse when wet and set like concrete when dry. Soils containing greater than 300 ppm magnesium are considered high magnesium soils as well soils with base saturations greater than 15 percent magnesium.

High magnesium soils can also occasionally show low potassium levels (less than 100 ppm). The addition of potassium fertilizers can actually make matters worse and cause even more dispersion of soil particles. Potassium, magnesium and sodium have similar properties. At high levels, potassium, magnesium and sodium cause a lack of structure by causing dispersion of soil particles, making soils drain poorly and become over saturated.

One solution for reducing high magnesium soil is applying limestone and/or gypsum in an effort to displace magnesium. Gypsum is calcium sulfate; the sulfate ion mobilizes the magnesium out of the root zone, with calcium taking its place. Adding a high calcium limestone can improve the structure of soil and quickly build calcium levels.

When attempting to improve soils with limestone, however, avoid dolomitic limestone, which is calcium magnesium carbonate; it can contain 30-50 percent magnesium. This will only escalate the magnesium levels in the soil. What little calcium is in dolomitic lime is not plant available. Our past and ongoing testing show that our high-calcium limestone is 4.5 times more plant available than dolomitic lime. Our brand is NatraSweet and others are also available in the marketplace.

A thorough soil analysis should be completed before applying limestone to high pH soils. Since pH has nothing to do with calcium, it only measures the hydrogen or hydroxide ion; a soil pH test cannot tell you if you have adequate calcium. In many cases with high pH soil, the high pH is due to magnesium hydroxide, sodium hydroxides, and potassium hydroxides. Adding high calcium lime adds primarily calcium. Calcium has stronger binding properties in the soil and will displace the weaker held sodium (or magnesium) cations from the clay particles. Clay particles can then bond together with the calcium to form a well structured soil.

It’s tough for many to accept that weeds grow because of certain soil conditions and not due to a lack of applied herbicides. Weeds are not the problem; they are the symptoms of problems. The origin of the word “weed” is “weod” meaning “little herb.” If herbs are intended for healing (think willow bark, or salicylic acid, which is chemically related to aspirin), then weeds are there to heal the soil. Weeds act as collectors of minerals. When they die and decay, the minerals from the plant are added back to the soil in a form available to other plants. The roots systems of many weeds can penetrate deep into the subsoil bringing up minerals and loosening it, making it possible for the root systems of less vigorous plants to follow.

I have had personal experience with this. I conducted a test plot on the edge of my lawn, looking to reduce a dandelion infestation without herbicides. The soil samples came back showing low organic matter, and low calcium. Observations were compaction six to eight inches deep, with little water infiltration and poor soil structure.

According to the book Weeds and Why They Grow, dandelions grow best in low-calcium soils. An application of our high-calcium NatraSweet resulted in a dramatic reduction in dandelions.

Fredrick Clements, an eminent botanist stated, “Each plant is an indicator.” In other words, the purpose of weeds is to correct soil problems. The common dandelion seems to thrive on bringing calcium back to the soil surface to become available as the plants decay. It seems only natural that by applying calcium to the soil we are telling the dandelion seeds to stay dormant, since their efforts to bring calcium to the soil surface are not needed.

Applying five to 10 pounds per 1000 sq ft. of high-calcium limestone per year can help prevent weeds such as dandelion and offer many other benefits for your lawn. Calcium will also help plants’ uptake of phosphorus, and nitrogen. It helps bacteria, fungi, protozoa and other soil life to cycle nutrients, which can reduce thatch. And for those going back to clover/grass lawns, the calcium helps clover stay healthy. Clover requires twice the calcium of grass and calcium necessary for clover nodulation, which supplies nitrogen.

Soil must be able to breathe to grow great grass. Calcium creates soil tilth and structure so that air and water can move more freely through soil. The added benefit of water moving into the soil is the reduced need for irrigation.

Many low-calcium soils can be improved with as little as two pounds per 1,000 square feet, while others may take up to 20 pounds per 1,000 square feet — but you won’t know for sure until you take a soil test. Read it and, if you don’t understand it, find a professional to help. Once you have a baseline, applications of a high-calcium limestone like NatraSweet or our gypsum product HydroSave should be the base of any organic program.

NatraTurf letters

Craig Dick, is the Blogronomist and Sales and Marketing Manager at NatraTurf. NatraTurf is a manufacturer of high quality organic products for the lawn and garden. Find other articles by Craig and guest writers at

About The Author

Paul Tukey

An international leader of the green movement, Mr. Tukey is a journalist, author, filmmaker, TV host, activist and award-winning public speaker, who is widely recognized as North America's leading advocate for landscape sustainability and toxic pesticide reduction strategies.

Number of Entries : 1023
  • Rosanne Aman

    Could this calcium be spread yet this year in WI? Could milorganite be applied at same time?

    Rosanne Aman

    • Paul Tukey

      Yes, absolutely. And, yes, Milorganite could be applied.

      • Linda Turner

        Just be careful with Milorganite. I’ve had customers tell me they’ve almost lost their dogs from eating it. Because it is derived from human waste, it is very high iron – hence, toxic to dogs. Also, don’t ever use it on edible crops.

  • JoeTurf

    A great article by Craig. The calcium:magnesium ratio is often misundertood or, at the very least, not preached often enough. Soil management and maintaining healthy soils is where it all starts. Thank, Craig, for reminding us.

    • Craig Dick

      Thanks for the comments Joe!

  • http://na Sharon Muczynski

    The process of making Milorganite also does not kill all viable pathogens, plus it can contain toxic metals.

  • thelawnblog

    Very powerful and insightful article. I was a lawn-care technician for a major operator in the northeast and I can tell you from first hand experience treating over 10,000 lawns over a 5 year period that the lawns that had adequate calcium were the ones that looked the best and resisted many diseases and insects compared to their non-calcium treated counterparts. I apply as much calcium (within reason) as I can and I can tell you the proof is in the soil. Thanks for a great place to be able to post a positive comment for the “good guys!”

  • John

    All my customer lawns,(800+)as per their soil tests recieve calcium 1-2 times a year. These lawns hardly ever have weeds or crabgrass! More promotion on this should be done. Forget “corn gluten”,

    • Paul Tukey

      You’re absolutely right. Lack of plant-available calcium is the cause of close to 90 percent of lawn weeds.

  • Craig Dick

    Thanks for all the comments!

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  • Larry Reid

    Will Calcium help to help to reduce the amount of clover in my lawn? I am trying reduce the amount.

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